The current invention relates to turbo-machines, such as steam and gas turbines. More specifically, the invention relates to an apparatus for controlling the clearance at the tips of the blades of such turbo-machines.
Typically, turbo-machines, such as gas and steam turbines, have a centrally disposed rotor that rotates within a stationary cylinder. The working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially outward from the periphery of the rotor shaft. The fluid imparts energy to the shaft that is used to drive a load, such as an electric generator or compressor. In order to ensure that as much energy as possible is extracted from the fluid, the radially outboard tips of the blades are closely encircled by a stationary ring, sometimes referred to as a "blade ring." From the standpoint of thermodynamic efficiency, it is desirable that the clearance between the blade tips and the stationary blade ring, typically referred to as the "tip clearance," be maintained at a minimum so as to prevent fluid from bypassing the row of blades.
Unfortunately, differential thermal expansion between the stationary cylinder and the rotor results in variations in the tip clearance with operating conditions. The specific effect of various operating conditions on tip clearance depends on the type of turbo-machine and its particular design--for example, tip clearances in gas turbine compressors often reach their minimum values during shutdown, whereas the tip clearances in low pressure steam turbines often reach their minimum values at steady state full load operation. Consequently, if insufficient tip clearance is provided at assembly, impact between the blade tips and the blade ring may occur when certain operating conditions are reached. Such impact can cause damage to the blades and is, therefore, to be avoided. Accordingly, a larger than desired tip clearance must be provided to ensure that there is adequate tip clearance to prevent the blade tip from contacting the stationary blade ring under all operating conditions.
Some turbo-machines employ conical tipped blades--that is, blades in which the tip lies in a plane that forms an acute angle with the center line of the rotor. In such cases the stationary blade ring also has a conical surface. Such conical tipped blades provide a number of advantages over cylindrical tipped blades, such as improved thermodynamic performance and simplified manufacture. However, the problem of controlling tip clearance is exacerbated in rotors using conical tipped blades. This is so because axial differential thermal expansion between the rotor and the cylinder during operation, as well as radial differential expansion, can result in a loss of tip clearance if the blade has a conical tip. As a result, much larger tip clearance variations are encountered in conical tipped blades. This situation is compounded in especially long rotors, such as those used in quadruple and sextuple flow low pressure steam turbines, since they have a long span over which axial expansion can build up.
One approach suggested for controlling tip clearance involves mounting the blade ring for radial movement in the stationary cylinder and using various mechanical mechanisms, such as screw threads or rings having inclined slots, to radially displace the blade ring as required to maintain tip clearance--see, for example U.S. Pat. No. 5,035,573 (Tseng et al.). However, this approach suffers from a variety of drawbacks. First, the mechanical mechanisms for displacing the blade rings are quite complicated and prone to sticking and other mechanical malfunctions. Second, such mechanical mechanisms are not adapted for rapid response so that contact between the blade tip and blade ring due to a sudden loss of tip clearance can occur if the operating conditions change rapidly--for example, due to an increase in condenser pressure or an overspeed condition or because the turbo-machine is suddenly tripped for safety reasons. Third, such mechanical mechanisms are not suited for the carefully controlled actuation necessary to continually fine tune the tip clearance during operation.
Another approach, disclosed in U.S. Pat. No. 4,844,688 (Clough et al.), utilizes a blade ring mounted for radial movement as discussed above, but employs air pressure to radially displace the blade ring by causing the pressurized air to deflect a flexible diaphragm that supports the blade ring. However, the amount of tip clearance adjustment that can be obtained by such elastic radial deflection is limited.
Accordingly, it would be desirable to provide an apparatus for controlling the tip clearance of a conical tipped blade that (i) provided for axial, as well as radial, displacement of the blade ring, (ii) allowed tip clearance to be continually and, if necessary, rapidly adjusted and (iii) was capable of displacing the blade ring by a large amount.